Unfortunately there are no switch-and-go or plug-and-play kits or parts that will modify a Razor Dirt Quad ATV to have a higher top speed and more torque.
An increase in speed and torque could be accomplished with a higher Voltage battery pack and controller, or a higher power motor and controller that runs on the original battery pack Voltage. All of these modification would require rewiring in the case of the battery pack and controller, or possibly redrilling the motor mounting holes or modifying the motor mounting plate in the case of upgrading the motor.
We have a Throttle Top Speed Adjustable Limiter with High/Low Speed Switch Connector that could be used in conjunction with a toggle switch, rocker switch, or key switch to limit the speed for the younger kids but open it up for the older one.
We have controllers and switches that allow for the motor to reverse so modifying the Dirt Quad to have reverse is definitely an option.
Please let us know if you have any questions.